It is possible for a hydrogen-absorbing alloy to absorb and desorb hydrogen reversely, and therefore a hydrogen-absorbing alloy is used for a heat pump, etc., in which an endothermic reaction caused in the process of desorbing hydrogen is used, as well as in an energy storage tank and in a nickel-hydrogen battery.
When a hydrogen-absorbing alloy is used, first, it is necessary to conduct an activation treatment on the hydrogen-absorbing alloy so that the absorbing and desorbing rate of hydrogen can be enhanced. Usually, the activation treatment is conducted as follows. While a container into which alloy powder is charged is being heated, air is drawn from the container. After that, absorbing and desorbing of hydrogen is repeated several times. Due to the foregoing operation, the activation treatment can be completed. However, the above treatment conditions and the times of treatment depend upon the type of hydrogen-absorbing alloy and the circumstances of the poisoning of the surface of alloy.
The process of producing a hydrogen-absorbing alloy usually includes melting, casting, heat-treating and crushing. In this case, crushing is conducted by a jaw crusher, roller crusher, etc., in two or three stages in accordance with the grain size of such hydrogen-absorbing alloy powder. For the prevention of oxidation of the surface of hydrogen-absorbing alloy powder, argon gas is frequently used as the atmosphere in this crushing process. When hydrogen-absorbing alloy powder is produced in the atmosphere of inert gas by means of mechanical crushing, gas atomization or hydrogenation-crushing, it has a clean surface. As long as the surface of hydrogen-absorbing alloy powder is clean, it is possible to conduct an activation treatment relatively easily. However, when the thus produced powder of a hydrogen-absorbing alloy is charged into a container for practical use, the hydrogen-absorbing alloy powder is exposed to air in many cases. At this time, the surface of the hydrogen-absorbing alloy powder is polluted with an oxide film, water and absorbed gases. For the above reasons, it becomes difficult to conduct the activation treatment. As a countermeasure to solve the above problems, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-240310 discloses a method in which hydrogen-absorbing alloy powder is finely crushed by an high compressive pressure-tight type roller mill in a nonoxidizing atmosphere. According to this method, it is possible to prevent oxidation of the surface of hydrogen-absorbing alloy powder in the process of crushing, however, it is impossible to enhance poisoning resistance after the completion of crushing, that is, it is impossible to protect the surface of hydrogen-absorbing alloy powder after the completion of crushing.
On the other hand, the following surface treatment methods for enhancing poisoning resistance of hydrogen-absorbing alloy powder have been disclosed. For example, Japanese Unexamined Patent Publication (Kokai) No. 7-207493 discloses a method in which electrolytic treatment is conducted in an alkaline solution containing fluorine ions while a hydrogen-absorbing alloy is used as a cathode. Also, Japanese Unexamined Patent Publication (Kokai) No. 62-167201 discloses a method in which hydrogen is absorbed into and desorbed from a hydrogen-absorbing alloy and then CO gas is adsorbed onto the surface of the hydrogen-absorbing alloy. However, the former method is not appropriate for surface treatment of a hydrogen-absorbing alloy containing aluminum or vanadium which tends to dissolve in an alkaline solution. Further, when the former method is adopted, it is necessary to provide an electrolytic processing tank, a vaporizing and solidifying apparatus used after electrolytic treatment and a plant for waste solution, which become factors which raise the production cost of producing the alloy.
When the latter treatment method is adopted, the hydrogen-absorbing alloy can be easily reactivated with drawing the vacuum and by absorbing and desorbing hydrogen gas. As described above, the poisoning resistance of the hydrogen-absorbing alloy has not reached a satisfactorily high level at present. On the other hand, the surface protection effect is not sufficient according to conditions of storage, as a result there is a problem in poisoning resistance. Therefore, the development of a surface treatment technique for enhancing the poisoning resistance of hydrogen-absorbing alloys has been desired.